Someone or something that is boring, uninspiring, lacking, or lame. Comes from the math/physics term null vector, the vector whose components are all equal to 0 (e.g. 0i + 0j + 0k). The null vector is movement in no direction--a lack of movement--hence its use to describe things that are not moving or inspiring. Can describe people, events, works of art, courses, etc.
"He's so lame. He's such a null vector."
"Good thing you cut school. The assembly was a null vector."